absorption line A feature identified in the spectrum of an object. It marks the absence of light at a specific wavelength, most frequently due to atoms in front or within the object. These atoms absorb the light and usually re-emit it into a new direction that is not exactly toward us. These spectral lines are used to measure the chemical composition, velocity, temperature, and motions of astronomical objects. Note that an absorption line is the silhouette of an emission line.

Angstrom A physical measure of distance named after the Physicist Anders J. Angstrom. There are 10 billion Angstomrs in 1 meter. A single molecule of water has a size of approximately 1 Angstrom. Astronomers tend to use Angstroms to measure the wavelength of light. Optical light (the light we see with our eyeball) has wavelengths ranging from 4000 to 7000 Angstroms.

color The property of an object associated with its visual appearance that is derived from the wavelengths of light that it reflects, emits, and absorbs. Our eyeballs are only sensitive to optical light, e.g. red, blue, green, yellow colors.

emission line Light emitted at a very precise wavelength. Physically, this light corresponds to an electron moving between two orbital levels around an atom. Similar to an absorption line, emission lines can be used to measure the chemical composition, velocity, temperature, and motions of astronomical objects. The example shown on the left is a the "Stingray" Planetary Nebula. Most of the color you see is due to emission lines of Oxygen and Hydrogen.

light Light is the energy associated with electromagnetic intereactions, for example an accelerated electron. Of course, we simply think of light as the 'stuff' that allows us to see objects in the universe. Light is not merely limited to the optical colors that our eyeballs observe but also includes gamma-rays, X-rays, radio waves, and microwaves. These various types of light only differ in their wavelength and therefore also their energy.

quasar A quasar is an extremely bright and distant object generally located at the center of a massive galaxy. Astronomers believe that quasars are powered by gas accreting onto a very massive black hole; many have as much mass as one billion Suns! These are among the brightest and most massive single objects in our universe. Quasars are almost certain to play a major role in the formation of galaxies, especially during the first several billion years of our universe. In addition, scientists study the light emitted by quasars to infer the properties of gas in the early universe.

redshift From a purely observational perspective, redshift refers to the apparent shift of the wavelength of light to larger values (i.e. to the red). This shift may occur because the source is moving away from the observer, because the space between the source and observer is expanding, or because the source is emitting within a strong gravitational field (e.g. near a black hole). Observers measure the redshift of light, especially absorption lines and emission lines to infer the velocity and/or distance of astronomical objects (stars, galaxies, quasars). Such measurements require one to obtain a spectrum of the source.

spectrum An evaluation of the brightness of the light emitted by an objects as a function of the wavelength of that light. For example, the Sun emits light a wide range of colors but the peak in its emission occurs at the green/yellow color which corresponds to a wavelength of approximately 5500 Angstroms. Many astronomical objects (stars, galaxies) emit or absorb a significant amount of light at a specific set of wavelengths producing absorption and emission lines. By obtaining a spectrum of these features, astronomers can measure the chemical composition, velocity, temperature, and motions of astronomical objects.